Princess Eugenie's father-in-law died days before her son's christening, according to reports.
George Brooksbank, 72, passed away last week, according to MailOnline.
He had suffered from Covid last year and had been unwell for some time, the publication said.
Eugenie and husband Jack Brooksbank baptised their son August on Sunday at All Saints Chapel in Great Windsor Park.
Zara and Mike Tindall's son Lucas was also christened at the ceremony, which was attended by the Queen.
A royal insider reportedly said: "George had been in hospital with Covid and had not been the same after that.
"He had been unwell for some time."
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Mr Brooksbank's cause of death has not been confirmed.
A retired chartered accountant and company director, he is said to have split his time with wife Nicola, 68, between London and France.
Buckingham Palace declined to comment on news of the death.
August, born in February, was meant to be christened in July before the event was cancelled due to the pandemic.
Another royal source reportedly said that it was "wonderful" for George to have been alive to see August born
The youngster is the first grandchild for Prince Andrew, 61, and Sarah, Duchess of York, 62.
He is also the ninth great-grandchild for the Queen, 95.
The monarch attended the ceremony on Sunday after she pulled out of a string of public events in recent weeks.
It had been reported that she was to make a decision on the day, having consulted her personal doctor.
Pictures then showed her being ferried away from the Berkshire chapel, with a lime green hat in view.
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Eugenie, 31, and Jack, 35, married in a lavish ceremony in 2018.
The christening was led by Canon Martin Poll, who is chaplain to Great Windsor Park and took place in the private Grade-II listed church which forms part of the Royal Lodge house.
It is thought it was the first time two royal babies were christened together.
Eugenie wore a silk white dress featuring embroidered detail, a headband and blue pendant necklace, while Jack wore a navy suit, crisp white shirt and light blue, patterned tie.
Zara Tindall, 40, whose son Lucas was born in March, opted for a black outfit and pearl dangling earrings as she drove herself to the venue.
Her husband, ex-rugby player Mike, 43, arrived in a separate vehicle dressed in a blazer.
The Queen has had to miss several events in recent weeks, including a scheduled appearance at the Cop26 climate summit in Glasgow and the Remembrance Sunday ceremony, due to a back sprain.
